.223 Remington vs. 5.56x45--Chambering and Throat Considerations
Is the .223 Remington the same as the 5.56x45? The answer is yes and no. There ARE differences between the .223 Remington as shot in civilian rifles and the 5.56x45 in military use. While the external cartridge dimensions are essentially the same, the .223 Remington is built to SAAMI specs, rated to 50,000 CUP max pressure, and normally has a shorter throat. The 5.56x45 is built to NATO specs, rated to 60,000 CUP max pressure, and has a longer throat, optimized to shoot long bullets. That said, there are various .223 Remington match chambers, including the Wylde chamber, that feature longer throats. Military 5.56x45 brass often, but not always, has thicker internal construction, and slightly less capacity than commercial .223 Rem brass.
Should you be worried about shooting 5.56x45 milspec ammo in a .223 Remington? The answer really depends on your chamber. 5.56 x45 ammo is intended for chambers with longer throats. If you shoot hot 5.56x45 ammo in short-throated SAAMI-spec chambers you can encounter pressure issues. The new long-throated 'Wylde' chamber allows safe use of military ammo. Wylde chambers are quite common in Rock River guns. Other manufacturers, such as Fulton Armory, offer modified "match chambers" with extended throats that allow safe use of 5.56x45 ammo in .223 Remington rifles. For a complete discussion of the .223 Rem vs. 5.56x45 question, read this Tech Notice from Winchester, and this GunZone Commentary by Dean Speir. Without belaboring the point, we'll repeat the official SAAMI position: "Chambers for military rifles have a different throat configuration than chambers for sporting firearms which, together with the full metal jacket of the military projectile, may account for the higher pressures which result when military ammunition is fired in a sporting chamber. SAAMI recommends that a firearm be fired only with the cartridge for which it is specifically chambered by the manufacturer."

.223 Remington vs. 5.56x45--Chambering and Throat Considerations
Is the .223 Remington the same as the 5.56x45? The answer is yes and no. There ARE differences between the .223 Remington as shot in civilian rifles and the 5.56x45 in military use. While the external cartridge dimensions are essentially the same, the .223 Remington is built to SAAMI specs, rated to 50,000 CUP max pressure, and normally has a shorter throat. The 5.56x45 is built to NATO specs, rated to 60,000 CUP max pressure, and has a longer throat, optimized to shoot long bullets. That said, there are various .223 Remington match chambers, including the Wylde chamber, that feature longer throats. Military 5.56x45 brass often, but not always, has thicker internal construction, and slightly less capacity than commercial .223 Rem brass.
Should you be worried about shooting 5.56x45 milspec ammo in a .223 Remington? The answer really depends on your chamber. 5.56 x45 ammo is intended for chambers with longer throats. If you shoot hot 5.56x45 ammo in short-throated SAAMI-spec chambers you can encounter pressure issues. The new long-throated 'Wylde' chamber allows safe use of military ammo. Wylde chambers are quite common in Rock River guns. Other manufacturers, such as Fulton Armory, offer modified "match chambers" with extended throats that allow safe use of 5.56x45 ammo in .223 Remington rifles. For a complete discussion of the .223 Rem vs. 5.56x45 question, read this Tech Notice from Winchester, and this GunZone Commentary by Dean Speir. Without belaboring the point, we'll repeat the official SAAMI position: "Chambers for military rifles have a different throat configuration than chambers for sporting firearms which, together with the full metal jacket of the military projectile, may account for the higher pressures which result when military ammunition is fired in a sporting chamber. SAAMI recommends that a firearm be fired only with the cartridge for which it is specifically chambered by the manufacturer."

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Thanks, but I was already aware of the differences between cartridges..... I'm asking specifically about Wylde chamber vs NATO, not .223 vs NATO.

merc1973,
As I understand the Wylde chamber, it's supposed to allow either/both cartridges to be used with equal accuracy. It's working for me with my RRA.

I own a Rock River Arms A4 with a SS 16" Varmint barrel that will shoot Winchester (white box) and UMC ammo with 45gr. JHP in a 1/2" group at 100 yards. I have yet to have a groundhog get in front of this weapon and not die.

Ofcourse, I have a free-float tube and a NM trigger too and that helps.

do not shoot 5.56 ammo in a rifle that is not chambered for it,you will run higher pressure,wear your match bbl out and the most companies will not warranty your bbl. check with the company. i shoot 69 gr match in my 5.56 rifle and it will feed 75 gr. have to watch oal, but the 1 in 9 twist not ideal for 75 gr. up to 200 yards you will not see a difference on target

do not shoot 5.56 ammo in a rifle that is not chambered for it,you will run higher pressure,wear your match bbl out and the most companies will not warranty your bbl. check with the company. i shoot 69 gr match in my 5.56 rifle and it will feed 75 gr. have to watch oal, but the 1 in 9 twist not ideal for 75 gr. up to 200 yards you will not see a difference on target

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